The present invention relates to positive electrodes for use in electrochemical energy storage devices. More particularly, the invention relates to lithium insertion positive electrodes containing either lithium nickel cobalt oxides or mixtures of such materials with other compounds.
Due to the increasing demand for battery-powered electronic equipment, there has been a corresponding increase in demand for rechargeable battery cells having high specific energies. In order to meet this demand, various types of rechargeable cells have been developed, including improved aqueous nickel-cadmium batteries, various formulations of aqueous nickel-metal hydride batteries and, recently, nonaqueous rechargeable lithium-ion cells (sometimes referred to as xe2x80x9clithium rocking chair,xe2x80x9d or xe2x80x9clithium intercalationxe2x80x9d cells). Lithium-ion cells are particularly attractive because they have a high cell voltage and a high specific energy.
Various positive electrodes (xe2x80x9ccathodesxe2x80x9d on discharge) have been studied and/or used in lithium ion batteries. These include lithium molybdenum sulfides, lithium molybdenum oxides, lithium vanadium oxides, lithium chromium oxides, lithium titanium oxides, lithium tungsten oxides, lithium cobalt oxides, lithium nickel oxides, and lithium manganese oxides. The preparation and use of lithium transition metal oxide positive electrodes are described in various publications including U.S. Pat. Nos. 4,302,518 and 4,357,215 issued to Goodenough et al., which are incorporated herein by reference for all purposes
While these materials, particularly lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium manganese oxide spinnel (LiMn2O4), have been found somewhat adequate, they each have some serious shortcomings. For example, they may have an unacceptably high irreversible capacity loss. This loss occurs during a first charge cycle when the cell""s negative electrode undergoes formation.
xe2x80x9cFormationxe2x80x9d refers to electrode modification processes employed after a cell is assembled, but before it is reversibly cycled; note that some but not all cell types require formation. In cells that require it, formation electrochemically modifies the cell""s electrodes so that thereafter they can be reversibly cycled. In lithium ion cells, formation involves an initial cycle which irreversibly drives some lithium ions from the positive electrode material to a carbon negative electrode (xe2x80x9canodexe2x80x9d on discharge) where they are believed to form a surface layer that has been found necessary to provide high energy cycling. This surface layer is known as a solid electrolyte interface or xe2x80x9cSEI.xe2x80x9d
The ratio of the first cycle charge capacity over the first cycle discharge capacity for a positive electrode is an important parameter in lithium ion cell design. This ratio should be compared to the same ratio for the cell""s negative electrode. In all cases, the positive electrode""s first cycle capacity ratio should be designed to match the negative electrode""s first cycle capacity ratio. If the positive electrode ratio exceeds the negative electrode ratio, lithium metal electroplating can occur, which can result in undesirable capacity fading and safety problems. In the case where this ratio is larger for the negative electrode, the cell""s reversible capacity is limited by the negative electrode. Likewise, when the opposite is true, the cell is limited by the positive electrode.
The problem of a positive electrode with a high first cycle ratio can be further understood by considering the example of lithium nickel oxide. As this material has a high first cycle charge ratio, less of it is required to xe2x80x9cformxe2x80x9d a given amount of carbon negative electrode than is required to reversibly cycle against that same amount of negative electrode (assuming that the negative electrode has a lower first charge ratio). Thus, if a cell is provided with an amount of lithium nickel oxide sufficient for formation, that cell will have insufficient lithium nickel oxide to utilize the available negative electrode material during subsequent reversible cycles. That is, the negative electrode will be underutilized, with some fraction of it constituting useless mass (which reduces the cell""s specific energy). On the other hand, if more lithium nickel oxide is used in the cell (beyond that required for formation), some metallic lithium will electroplate onto the negative electrode during formation, presenting the danger that the electroplated lithium metal will undergo an exothermic chemical reaction.
By designing a mixed oxide to include nickel plus another metal which tends to equalize the amount of oxide required to reversibly cycle against and form a given amount of negative electrode material, the above difficulties can be mitigated. Lithium nickel cobalt oxides are potentially useful candidates for such applications because the presence of cobalt does, in fact, tend to equalize the amount of oxide required for these two functions. Note that, in contrast to lithium nickel oxide, more lithium cobalt oxide is required to form a negative electrode than to reversibly cycle against it. Thus, it intuitively follows that the presence of cobalt in a nickel oxide will tend to match the formation capacity and reversible capacity of the oxide.
The presence of cobalt has another advantage. During reversible cell cycling, it reduces the average oxidation state of transition metals in the oxide lattice. A fresh uncycled positive electrode could have the formula LiMO2, with the valence of M being equal to 3. On fall charge, the positive electrode oxide could in theory have a formula MO2, with the valence of M being equal to 4. Thus, during charge the transition metal""s oxidation state increases, and during discharge the transition metal""s oxidation state decreases. Because some lithium in the positive electrode is irreversibly lost during the formation cycle, the positive electrode matrix can never discharge the whole way to its initial stoichiometry of LiMO2. As a consequence, the valence of M is never lowered all the way to 3 (during normal reversible cycling). Rather, the oxidation state of M is bracketed between a value greater than 3 and a value lower than 4 during reversible cycling. The bounds of this oxidation state are determined by how much lithium is lost during formation.
Obviously, compounds with high irreversible capacity losses will cycle at higher average oxidation states of M than compounds with lower irreversible capacity losses. Lithium nickel oxide has a much higher irreversible capacity loss (about 40 mAxc2x7hr/gm) than lithium cobalt oxide (about 8 mAxc2x7hr/gm). The introduction of cobalt into the LiNiO2 matrix reduces the irreversible capacity loss and thereby reduces the oxidation state of M during reversible cycling. As a consequence, the oxide lattice is at a lower oxidation state and therefore less reactive and less likely to pose a significant safety risk.
Nevertheless, LiNiaCObO2 compounds still pose the risk of decomposing on severe overcharge. Such decomposition reaction is accompanied by a release of oxygen and energy which increases the cell""s internal temperature and pressure, and thereby increases the risk of igniting the electrolyte. Obviously, designs that avoid these potential problems will be of significant commercial importance.
Note that the lithium cobalt oxide and lithium nickel oxide also may undergo a decomposition reaction on overcharge. Generally, it is known that LiCoO2, LiNiO2 and LiMn2O4 have varying degrees of thermal stability in their delithiated forms (see, e.g., J. R. Dahn et al., Solid State Ionics 69 (1994) 265-270). For example, it is known that the layered compound LiNi0.5O2 is transformed to the spinnel LiNi2O4 on heating to above 200xc2x0 C. This transformation is accompanied by little mass loss or heat generation. In contrast, at higher degrees of delithiation (e.g., Li0.3NiO2), the transformation to spinnel is also accompanied by significant oxygen generation and heat liberation. Delithiated LiCoO2 does not undergo a transformation to spinnel form, but rather decomposes to layered LiCoO2 and stable Co3O4 at about 245xc2x0 C. Some oxygen is also released in this reaction.
Still other problems remain in many lithium metal oxide positive electrodes. For example, many lithium metal oxides (e.g., LiMn2O4, LiCoO2, LiNiO2, and some atomic mixtures of Mn, Co, and Ni oxides) have substantially flat discharge profiles. That is, their voltage varies only slightly with state of charge until very nearly all of their capacity has been exhausted. Thus, from full charge until nearly complete discharge (during which time most available lithium enters the positive electrode), the electrode voltage remains high and nearly constant. Only when most available lithium has been extracted from positive electrode (at the end of discharge) does it exhibit a characteristic sharp drop in voltage. While such discharge characteristics provide high and relatively constant potentials during most of discharge, they can cause cells to perform poorly at high rates of discharge. This results because ohmically caused variations of potential within the electrode can not be compensated by variations in the reaction rate. Thus, the electrode material is under-utilized at any given state of discharge, thereby limiting the rate, energy, and cycling performance of cells, as discussed in T. Fuller et al. J. Electrochem. Soc., 1, 114 (1994), incorporated herein by reference for all purposes.
In view of the above, there is a need for improved lithium insertion positive electrode materials which have substantially matched formation and reversible cycling capacities, resist decomposition on overcharge, and have sloping discharge profiles.
The present invention provides improved positive electrode materials containing compounds of the following formula: LixNiyCOzMnO2, where M is selected from the group consisting of aluminum, titanium, tungsten, chromium, molybdenum, magnesium, tantalum, silicon, and combinations thereof, x is between about 0 and about 1 and can be varied within this range by electrochemical insertion and extraction, the sum of y+z+n is about 1, n ranges between above 0 to about 0.25, y and z are both greater than 0, and the ratio z/y ranges from above 0 to about 1/3. Preferably, the compound has an xcex1-NaCrO2 crystal structure.
These materials have the advantage of maintaining capacity better than lithium nickel oxide on repeated cycling. They also have the advantage of being relatively safe in comparison to lithium nickel oxide and lithium cobalt oxide. It is believed that their conductivity is reduced on overcharge (i.e., at low lithium contents), thereby preventing current from continuing to drive detrimental overcharge reactions.
The lithium nickel cobalt metal oxides of this invention may be prepared by various routes. In one preferred embodiment, they are prepared as follows: combining (i) a lithium composition including at least one of lithium carbonate, lithium hydroxide, lithium acetate, and lithium nitrate, (ii) at least one of a metal hydroxide, a metal oxide, an elemental metal, or a metal carbonate containing M, (iii) a cobalt composition including at least one of cobalt oxide, cobalt hydroxide, cobalt carbonate, cobalt acetate, and cobalt nitrate, and (iv) a nickel composition including at least one of nickel hydroxide, nickel carbonate, nickel acetate, and nickel nitrate; and (b) thermally reacting the combination of (i), (ii), (iii) and (iv) at a temperature of between about 500 and 1300xc2x0 C., more preferably at about 600 to 1000xc2x0 C., and most preferably at about 750 to 850xc2x0 C.
The compounds of this invention may be used in mixture with one or more other compounds to form composite positive electrodes having high capacity, long cycle life, and good safety. Preferably the mixture includes a lithium manganese oxide compound in addition to the lithium nickel cobalt metal oxide. More preferably, the lithium manganese oxide compound is lithium manganese oxide (LiMn2O4) or a lithium manganese metal oxide such as LiMn2xe2x88x92rM1rO4, where M1 is tungsten, titanium, chromium, nickel, cobalt, iron, tin, zinc, zirconium, silicon, or a combination thereof, and r ranges between about 0 and 1. Preferably, r ranges from about 0 to 0.08. In a particularly preferred embodiment, the mixture includes one of the above two listed manganese compounds together with a lithium nickel cobalt aluminum oxide (e.g., LiNi0.6Co0.15Al0.25O2).
In a related embodiment, the positive electrode includes a mixture of lithium nickel cobalt oxide (containing no aluminum or other non-lithium metal) together with LiMn2xe2x88x92rM1rO4.
Yet another aspect of the invention provides a lithium ion cell which may be characterized as including (a) a cell container; (b) a negative electrode provided within the cell container and capable of intercalating lithium during charge and deintercalating lithium during discharge; (c) an electrolyte conductive to lithium ions and provided within the cell container; and (d) a positive electrode provided within the cell container and capable of taking up lithium on discharge and releasing lithium on charge. The positive electrode includes a compound or mixture of compounds as described above. Preferably the negative electrodes in such cells include a mixture of homogeneous graphitic carbon particles and homogeneous non-graphitic carbon particles. When such mixed carbon negative electrodes are employed, it will be particularly desirable to employ a positive electrode of lithium nickel cobalt oxide admixed with lithium manganese metal oxide. Further, the electrolytes in cells of this invention preferably include a mixture of ethylene carbonate, diethyl carbonate, and dimethyl carbonate with a dissolved lithium containing salt such as at least about 0.8 molar LiPF6, LiBF4, or LiN(SO2C2F5)2. In an especially preferred embodiment, the electrolyte includes about 1.39 molar LiPF6. Other electrolytes may include one of the following solvent mixtures: (1) ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate; (2) ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate; and (3) ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate. The electrolyte may also include a polymer or gelling agent
These and other features of the present invention will be presented in more detail in the following specification of the invention and in the figures.